The present invention relates to a fuel cell, and more particularly to a type of fuel cell that generates electric power by feeding organic liquid fuel such as methanol directly to an anode.
Portable electronic equipment such as cell phones, personal digital assistants, notebook-sized personal computers, portable audios and portable visuals has been becoming popular. Such portable electronic equipment is conventionally driven by primary batteries or secondary batteries. Particularly, Nickel-Cadmium batteries or lithium-ion batteries are used as the secondary batteries, and development of downsized batteries with high-energy density have been developed. However, since the secondary batteries need to be charged for a specified period of time after consumption of a specific amount of electric power, the batteries supporting continuous long-run driving with short charging time have been demanded.
In order to meet the demand, fuel cells operated without charging have been proposed. The fuel cells are generators for electrochemically converting chemical energy of fuel to electric energy. One such fuel cell is a Polymer Electrolyte Fuel Cell (PEFC) for generating electric power with use of a perfluorocarbon sulfonic acid-based electrolyte to reduce hydrogen gas in an anode and to reduce oxygen in a cathode. Such PEFC has a feature of batteries with high power density, and its development is being pursued.
However, hydrogen gas used in such PEFC is low in volume energy density, which requires the volume of a fuel tank to be enlarged. Further, the PEFC needs to be equipped with auxiliary equipment such as devices to feed fuel gas and oxide gas to a fuel cell body (generation section) and humidifiers for stabilizing battery performance, which increases the size of a fuel cell system. Therefore, the PEFC is not suitable as a power source for portable electronic equipment.
A Direct Methanol Fuel Cell (DMFC) for generating electric power by directly extracting protons from methanol, although having a disadvantage in that its output is smaller than that of the PEFC, has advantages in that the volume energy density of fuel can be increased and auxiliary equipment of the fuel cell body can be reduced, which allows downsizing. Because of this reason, the DMFC is drawing attention and several proposals regarding the DMFC have been made.
The reaction on the anode side and the reaction on the cathode side which takes place inside the fuel cell body of the DMFC are as follows:
Anode side: CH3OH+H2O→6H++6e−+CO2 
Cathode side: 6H++6e−+3/2O2→3H2O
As shown in the above chemical formulas, power generation with use of the fuel cell leads to generation of carbon dioxide (CO2) on the anode side and water (H2O) on the cathode side. Consequently, in order to carry out continuous power generation, it is necessary to establish a fuel cell system including auxiliary equipment for treating the produced carbon dioxide and water.
An example of the structure of such conventional DMFC-type fuel cells is found in U.S. Pat. No. 5,599,638 (FIG. 1 and FIG. 2). This fuel cell system adopts a fuel circulation system in which a pump is used for stably feeding methanol solution to an anode from a circulation tank containing the methanol solution as a fuel, and unconsumed methanol solution remaining in the anode is returned to the circulation tank to be recycled as a fuel.
The water (H2O) produced by power generation on the cathode side is collected by a water collector and sent to the circulation tank containing methanol solution.
Generally, the fuel cell body is often constituted such that fuel and oxygen passageways in the anode and the cathode would be lengthened so as to prolong residence time of fuel and oxygen in the anode and the cathode for increasing the reaction of fuel and oxygen fed to the anode or the cathode. For example, a separator is used which is constituted of a conductive material such as carbon materials or the like, and is composed of one inlet, one outlet, and a plurality of grooves meandering between the upper end and the lower end.